U.S. patent number 6,413,584 [Application Number 09/522,876] was granted by the patent office on 2002-07-02 for method for preparing a gas turbine airfoil protected by aluminide and platinum aluminide coatings.
This patent grant is currently assigned to General Electric Company. Invention is credited to Jonathan P. Clarke, Antonio F. Maricocchi, Roger D. Wustman.
United States Patent |
6,413,584 |
Wustman , et al. |
July 2, 2002 |
Method for preparing a gas turbine airfoil protected by aluminide
and platinum aluminide coatings
Abstract
A gas turbine component article has an airfoil section and is
formed of a nickel-base superalloy. An unmasked region of the
airfoil section has a platinum aluminide protective coating, and a
masked region of the airfoil section has an aluminide coating. The
platinum aluminide preferably is deposited at a trailing edge of
the airfoil section that is susceptible to low-cycle fatigue damage
when a platinum aluminide coating is present.
Inventors: |
Wustman; Roger D. (Loveland,
OH), Maricocchi; Antonio F. (Loveland, OH), Clarke;
Jonathan P. (West Chester, OH) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
23471689 |
Appl.
No.: |
09/522,876 |
Filed: |
March 10, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
373269 |
Aug 11, 1999 |
6254756 |
|
|
|
Current U.S.
Class: |
427/250; 427/252;
427/272; 427/255.7 |
Current CPC
Class: |
C23C
28/021 (20130101); C25D 7/00 (20130101); C23C
28/028 (20130101); C23C 28/023 (20130101); C23C
28/325 (20130101); C23C 28/322 (20130101); C23C
28/3455 (20130101); C25D 5/02 (20130101); C23C
10/58 (20130101); F01D 5/288 (20130101); C23C
28/321 (20130101); C23C 4/01 (20160101); Y10T
428/12875 (20150115); Y02T 50/67 (20130101); Y10T
428/24802 (20150115); Y02T 50/671 (20130101); Y02T
50/60 (20130101); F05D 2300/143 (20130101); F05D
2230/90 (20130101); Y10T 428/12736 (20150115); F05D
2300/611 (20130101) |
Current International
Class: |
F01D
5/28 (20060101); C23C 10/00 (20060101); C23C
10/58 (20060101); C25D 5/02 (20060101); C25D
7/00 (20060101); C23C 4/00 (20060101); C23C
28/00 (20060101); C23L 016/12 () |
Field of
Search: |
;427/250,252,255.7,272 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chen; Bret
Attorney, Agent or Firm: Narciso; David L. Garmong; Gregory
O.
Parent Case Text
This application is a continuation-in-part of application Ser. No.
09/373,269, filed Aug. 11, 1999, now U.S. Pat. No. 6,254,756 for
which priority is claimed.
Claims
What is claimed is:
1. A method for preparing an article protected by an aluminide
coating, comprising:
furnishing a component article of a gas turbine having an airfoil
section, the component comprising a nickel-base superalloy;
masking a masked portion of the airfoil section, leaving an
unmasked portion of the airfoil section;
depositing a noble metal onto the airfoil section as a substrate,
the result being the unmasked portion having the noble metal layer
thereon and the masked portion having no noble metal layer
thereon;
removing the mask; and
depositing an aluminum-containing layer onto the airfoil section of
the article.
2. The method of claim 1, wherein the noble metal is selected from
the group consisting of platinum, palladium, and rhodium.
3. The method of claim 1, wherein the noble metal is platinum.
4. The method of claim 1, including an additional step, conducted
at least partially concurrently with the step of depositing an
aluminum-containing layer, of
interdiffusing the noble metal, the aluminum-containing layer, and
the substrate.
5. The method of claim 1, wherein the masked portion is at a
trailing edge of the airfoil section.
6. The method of claim 1, wherein the masked portion is at a
trailing edge adjacent to a platform of the airfoil section.
7. The method of claim 1, including an additional step, prior to
the step of masking, of
identifying an area of the article that is potentially subject to
mechanical property degradation in the presence of a coating, and
wherein the masked portion includes the area of the airfoil section
that is potentially subject to the mechanical property
degradation.
8. The method of claim 1, including an additional step, after the
step of depositing an aluminum-containing layer, of
depositing a ceramic layer overlying the aluminum-containing
layer.
9. A method for preparing an article protected by an aluminide
coating, comprising:
furnishing a component article of a gas turbine having an airfoil
section, the component comprising a nickel-base superalloy;
identifying an area of the airfoil section that is potentially
subject to mechanical property degradation in the event that a
coating is present;
masking a masked portion of the airfoil section which includes the
area that is potentially subject to mechanical property
degradation, leaving an unmasked portion of the airfoil
section;
depositing a layer of platinum onto the airfoil section as a
substrate, the result being the unmasked portion having the
platinum layer thereon and the masked portion having no platinum
layer thereon;
removing the mask; and
depositing an aluminum-containing layer onto the airfoil section of
the article.
10. The method of claim 9, including an additional step, conducted
at least partially concurrently with the step of depositing an
aluminum-containing layer, of
interdiffusing the platinum layer, the aluminum-containing layer,
and the substrate.
11. The method of claim 9, wherein the masked portion is at a
trailing edge of the airfoil section.
12. The method of claim 9, wherein the masked portion is at a
trailing edge adjacent to a platform of the airfoil section.
13. The method of claim 9, including an additional step, after the
step of depositing an aluminum-containing layer, of
depositing a ceramic layer overlying the aluminum-containing
layer.
14. A method for preparing an article protected by an aluminide
coating, comprising:
furnishing a component article of a gas turbine having an airfoil
section, the component comprising a nickel-base superalloy;
applying a mask to a portion of the airfoil section so that there
is a masked portion and an unmasked portion of the airfoil
section;
depositing a noble metal onto the masked portion and the unmasked
portion of the airfoil section, the result being the unmasked
portion having the noble metal layer thereon and the masked portion
having no noble metal layer thereon;
removing the mask; and
depositing an aluminum-containing layer onto the airfoil section of
the article so that the masked portion has the aluminum-containing
layer deposited thereon and the unmasked portion has both the noble
metal layer and the aluminum-containing layer deposited
thereon.
15. The method of claim 14, wherein the noble metal is selected
from the group consisting of platinum, palladium, and rhodium.
16. The method of claim 14, wherein the noble metal is
platinum.
17. The method of claim 14, including an additional step, conducted
at least partially concurrently with the step of depositing an
aluminum-containing layer, of
interdiffusing the noble metal, the aluminum-containing layer, and
the nickel-base alloy of the article.
18. The method of claim 14, wherein the masked portion is at a
trailing edge of the airfoil section.
19. The method of claim 14, including an additional step, prior to
the step of applying a mask, of
identifying an area of the article that is potentially subject to
mechanical property degradation in the presence of a coating, and
wherein the masked portion includes the area of the airfoil section
that is potentially subject to the mechanical property
degradation.
20. The method of claim 14, including an additional step, after the
step of depositing an aluminum-containing layer, of
depositing a ceramic layer overlying the aluminum-containing layer.
Description
This invention relates to protective coatings on articles, and,
more particularly, to aluminide and platinum-aluminide coatings on
aircraft gas turbine components having airfoils.
BACKGROUND OF THE INVENTION
In an aircraft gas turbine (jet) engine, air is drawn into the
front of the engine, compressed by a shaft-mounted compressor, and
mixed with fuel. The mixture is combusted, and the resulting hot
combustion gases are passed through a turbine mounted on the same
shaft. The flow of gas turns the turbine by contacting an airfoil
portion of the turbine blade, which turns the shaft and provides
power to the compressor. The hot exhaust gases flow from the back
of the engine, driving it and the aircraft forwardly.
The hotter the turbine gases, the more efficient is the operation
of the jet engine. There is thus an incentive to raise the turbine
operating temperature. However, the maximum temperature of the
turbine gases is normally limited by the materials used to
fabricate the turbine vanes and turbine blades of the turbine. In
current engines, the turbine vanes and blades are made of
nickel-based or cobalt-based superalloys that can operate at
temperatures of up to 1900-2100.degree. F.
Many approaches have been used to increase the operating
temperature limits and operating lives of the airfoils of the
turbine blades and vanes. The compositions and processing of the
materials themselves have been improved. The articles may be formed
as oriented single crystals to take advantage of superior
properties observed in certain crystallographic directions.
Physical cooling techniques are used. For example, internal cooling
channels may be provided within the components, and cooler air is
forced through the channels during engine operation.
In another approach, a protective layer is applied to the airfoil
of the turbine blade or turbine vane component, which acts as a
substrate. Among the currently known diffusional protective layers
are aluminide and platinum aluminide layers. The protective layer
protects the substrate against environmental damage from the hot,
highly corrosive combustion gases. This protective layer, with no
overlying ceramic layer, is useful in intermediate-temperature
applications. For higher temperature applications, a ceramic
thermal barrier coating layer may be applied overlying the
protective layer, to form a thermal barrier coating (TBC) system.
The ceramic thermal barrier coating layer insulates the component
from the exhaust gas, permitting the exhaust gas to be hotter than
would otherwise be possible with the particular material and
fabrication process of the substrate.
Even with the use of these protective techniques, there remain
problems to overcome in extending the operating service
temperatures and operating lives of the turbine blade components.
For example, some portions of the airfoil have been observed to
fail prematurely due to low-cycle fatigue, wherein that portion of
the airfoil is subjected to repetitive, relatively large strain
cycles at elevated temperature. There is a need for an approach to
overcoming such problems, while retaining the benefits of the
available protection techniques.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a technique for reducing the
susceptibility of gas turbine components to property degradation
such as low-cycle fatigue failures, while retaining the benefits
associated with protective coatings that are applied to the
components. The present approach takes a highly selective approach
to the protection of the turbine components, optimizing the
performance of the protective system at various portions of the
component. Expensive platinum is conserved, although this is a
relatively minor benefit. The present approach may be accomplished
as part of the normal production operation, without major
modifications.
A method for preparing an article protected by an aluminide coating
is utilized with a component article of a gas turbine having an
airfoil section and comprising a nickel-base superalloy. The method
includes masking a portion of the airfoil section, leaving an
unmasked portion of the airfoil section, and depositing a noble
metal such as platinum onto the airfoil section as a substrate. The
result is that the unmasked portion has the noble metal layer
thereon and the masked portion has no noble metal layer thereon.
The mask is removed, and an aluminum-containing layer is deposited
onto the airfoil section of the article. Typically the noble metal,
the aluminum-containing layer, and the substrate material are
interdiffused. A ceramic layer may be deposited over the
aluminum-containing layer, to form a thermal barrier coating.
In the preferred application of the present invention, the
selective use of the noble metal allows a reduction in premature
failures due to low-cycle fatigue. The application is based upon
the recognition that platinum enhances the attraction of aluminum
to the coating. For a part coated in an atmosphere of constant
aluminum activity, the area with platinum will coat thicker and
have a higher total aluminum content than an area without platinum.
Thick coatings are more prone to mechanical property degradation
such as low-cycle-fatigue cracking. The incidence of low-cycle
fatigue damage may be lessened in some areas of the airfoil by
using an aluminide protective coating rather than a platinum
aluminide protective coating.
Consistent with this approach, an area of the article that is
subject to mechanical property degradation such as low-cycle
fatigue damage is identified. The masked portion includes the area
of the article that is subject to such mechanical property
degradation in the form of low-cycle fatigue damage. A region of
particular concern is the portion of the airfoil adjacent to a
trailing edge of the airfoil, and most particularly the trailing
edge adjacent to a platform portion of the turbine component. The
trailing edge region adjacent to the platform experiences less
severe temperatures than the leading edge-region, so the use of the
aluminide coating at the trailing edge root is sufficient from a
protection standpoint.
The result is an article protected by an aluminide coating,
comprising a nickel-base superalloy substrate in the form of a gas
turbine component article having an airfoil section, a platinum
aluminide coating in an unmasked portion of the airfoil, and an
aluminide coating in a masked portion and in the unmasked portion
of the airfoil. The masked portion is the portion of the airfoil
that is identified as most susceptible to mechanical property
degradation in the form of low-cycle fatigue damage. In the case of
most interest, this portion is the trailing edge root of the
airfoil, as discussed above.
The present approach thus provides a technique for selectively
coating the gas turbine component to protect the component yet to
reduce the incidence of property degradation such as low-cycle
fatigue failures. Other features and advantages of the present
invention will be apparent from the following more detailed
description of the preferred embodiment, taken in conjunction with
the accompanying drawings, which illustrate, by way of example, the
principles of the invention. The scope of the invention is not,
however, limited to this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the convex side of a turbine
blade;
FIG. 2 is a perspective view of the concave side of the turbine
blade;
FIG. 3 is a block flow diagram of a preferred method for practicing
the invention; and
FIG. 4 is a schematic enlarged sectional view through the airfoil
of the turbine blade, taken along line 4--4 of FIG. 2 and
illustrating both the region coated with platinum aluminide and the
region coated with an aluminide.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 and 2 depict a component article of a gas turbine engine
such as a turbine blade or turbine vane, and in this illustration a
turbine blade 20. FIG. 1 is the view of the convex side, and FIG. 2
is the view of the concave side. The turbine blade 20 includes an
airfoil 22 against which the flow of hot exhaust gas is directed.
(The turbine vane has a similar appearance in respect to the
pertinent airfoil portion.) The turbine blade 20 is mounted to a
turbine disk (not shown) by a dovetail 24 which extends downwardly
from the airfoil 22 and engages a slot on the turbine disk. A
platform 26 extends longitudinally outwardly from the area where
the airfoil 22 meets the dovetail 24. In some articles, a number of
cooling channels extend through the interior of the airfoil 22,
ending in openings 28 in the surface of the airfoil 22. A flow of
cooling air is directed through the cooling channels, to reduce the
temperature of the airfoil 22.
As illustrated, the airfoil 22 portion of the turbine blade 20 is
curved in an airfoil shape. There is a concavely curved side,
termed the concave side 30 (also sometimes known as the "pressure"
side of the airfoil), and a convexly curved side, termed the convex
side 32 (also sometimes known as the "suction" side of the
airfoil). A curved leading edge 34 separates the concave side 30
from the convex side 32 along one longitudinal margin of the
airfoil 22. A more sharply defined trailing edge 36 separates the
concave side 30 from the convex side 32 along the other
longitudinal margin of the airfoil 22. The airfoil 22 terminates in
a tip 38 remote from the dovetail 24. In service, the pressurized
hot combustion gas from the combustors is directed against the
concave side 30. This concave side 30 therefore requires more
protection against the incident hot combustion gas than does the
convex side 32. To provide this protection, at least a portion of
the concave side 30 is coated with a protective coating, either in
the form of an environmental coating or a thermal barrier
coating.
FIG. 3 depicts an approach for preparing an article such as the
turbine blade 20. The component article is furnished, numeral 50,
in the preferred case the turbine blade 20 or a turbine vane. The
component article includes the airfoil 22. The component article is
made of a nickel-base superalloy. "Nickel base" means that the
article has more nickel than any other element, on a weight
percentage basis. The superalloy typically includes gamma-prime
forming elements, and is strengthened by the precipitation of
gamma-prime (nominally Ni.sub.3 (Al,Ti)) phase in a gamma nickel
solid solution matrix.
Any operable material may be used as the substrate article. The
preferred nickel-base superalloy has a composition, in weight
percent, of from about 4 to about 20 percent cobalt, from about 1
to about 10 percent chromium, from about 5 to about 7 percent
aluminum, from 0 to about 2 percent molybdenum, from about 3 to
about 8 percent tungsten, from about 4 to about 12 percent
tantalum, from 0 to about 2 percent titanium, from 0 to about 8
percent rhenium, from 0 to about 6 percent ruthenium, from 0 to
about 1 percent niobium, from 0 to about 0.1 percent carbon, from 0
to about 0.01 percent boron, from 0 to about 0.1 percent yttrium,
from 0 to about 1.5 percent hafnium, balance nickel and incidental
impurities.
A most preferred alloy composition is Rene' N5, which has a nominal
composition in weight percent of about 7.5 percent cobalt, about 7
percent chromium, about 6.2 percent aluminum, about 6.5 percent
tantalum, about 5 percent tungsten, about 1.5 percent molybdenum,
about 3 percent rhenium, about 0.05 percent carbon, about 0.004
percent boron, about 0.15 percent hafnium, up to about 0.01 percent
yttrium, balance nickel and incidental impurities. Other operable
superalloys include, for example, Rene' N6, which has a nominal
composition in weight percent of about 12.5 percent cobalt, about
4.2 percent chromium, about 1.4 percent molybdenum, about 5.75
percent tungsten, about 5.4 percent rhenium, about 7.2 percent
tantalum, about 5.75 percent aluminum, about 0.15 percent hafnium,
about 0.05 percent carbon, about 0.004 percent boron, about 0.01
percent yttrium, balance nickel and incidental impurities; Rene'
142, which has a nominal composition in weight percent of about 6.8
percent chromium, 12.0 percent cobalt, 1.5 percent molybdenum, 2.8
percent rhenium, 1.5 percent hafnium, 6.15 percent aluminum, 4.9
percent tungsten, 6.35 percent tantalum, 150 parts per million
boron. 0.12 percent carbon, balance nickel and incidental
impurities; CMSX-4, which has a nominal composition in weight
percent of about 9.60 percent cobalt, about 6.6 percent chromium,
about 0.60 percent molybdenum, about 6.4 percent tungsten, about
3.0 percent rhenium, about 6.5 percent tantalum, about 5.6 percent
aluminum, about 1.0 percent titanium, about 0.10 percent hafnium,
balance nickel and incidental impurities; CMSX-10, which has a
nominal composition in weight percent of about 7.00 percent cobalt,
about 2.65 percent chromium, about 0.60 percent molybdenum, about
6.40 percent tungsten, about 5.50 percent rhenium, about 7.5
percent tantalum, about 5.80 percent aluminum, about 0.80 percent
titanium, about 0.06 percent hafnium, about 0.4 percent niobium,
balance nickel and incidental impurities; PWA1480, which has a
nominal composition in weight percent of about 5.00 percent cobalt,
about 10.0 percent chromium, about 4.00 percent tungsten, about
12.0 percent tantalum, about 5.00 percent aluminum, about 1.5
percent titanium, balance nickel and incidental impurities;
PWA1484, which has a nominal composition in weight percent of about
10.00 percent cobalt, about 5.00 percent chromium, about 2.00
percent molybdenum, about 6.00 percent tungsten, about 3.00 percent
rhenium, about 8.70 percent tantalum, about 5.60 percent aluminum,
about 0.10 percent hafnium, balance nickel and incidental
impurities; and MX-4, which has a nominal composition as set forth
in U.S. Pat. No. 5,482,789, in weight percent, of from about 0.4 to
about 6.5 percent ruthenium, from about 4.5 to about 5.75 percent
rhenium, from about 5.8 to about 10.7 percent tantalum, from about
4.25 to about 17.0 percent cobalt, from 0 to about 0.05 percent
hafnium, from 0 to about 0.06 percent carbon, from 0 to about 0.01
percent boron, from 0 to about 0.02 percent yttrium, from about 0.9
to about 2.0 percent molybdenum, from about 1.25 to about 6.0
percent chromium, from 0 to about 1.0 percent niobium, from about
5.0 to about 6.6 percent aluminum, from 0 to about 1.0 percent
titanium, from about 3.0 to about 7.5 percent tungsten, and wherein
the sum of molybdenum plus chromium plus niobium is from about 2.15
to about 9.0 percent, and wherein the sum of aluminum plus titanium
plus tungsten is from about 8.0 to about 15.1 percent, balance
nickel and incidental impurities. The use of the present invention
is not limited to these preferred alloys, and has broader
applicability.
An area to be masked is identified. In the illustrated preferred
approach, this identification is made on the basis of
considerations of mechanical property degradation such as potential
low-cycle fatigue susceptibility of regions coated with a platinum
aluminide coating, numeral 52. The inventors have observed that
certain areas of the turbine blade that are coated with a platinum
aluminide coating are potentially susceptible to mechanical
property degradation in the form of low-cycle fatigue damage. These
areas include a portion 40 of the concave side 30 near the trailing
edge 36. The problem is most acute in a root portion 42 of the
concave side near the trailing edge 36, and adjacent to the top
side of the platform 26. The potential susceptibility to low-cycle
fatigue damage arises when the portion 40 and/or the root portion
42 are coated with a platinum aluminide coating, but is minimized
by an aluminide coating that contains little if any platinum or
other noble metal. Accordingly, the portions 40 and/or 42 are
identified as the areas to be masked.
Other criteria may be used instead in particular situations. For
example, it may be necessary to cover the ends of the opening 28 to
prevent deposition therein and into the internal passages.
A masked portion 44 of the airfoil is masked, numeral 54. As noted,
in the preferred approach the masked portion is preferably either
the portion 40 or the root portion 42. Masking is accomplished by
any operable technique that will prevent the deposition of platinum
into the masked portion. The deposition of platinum is preferably
accomplished by an electrodeposition process as will be described
subsequently, and any operable masking technique that will prevent
deposition of platinum on the masked portion 44 may be used. For
example, the masked portion 44 may be covered with a physical mask
such as an illustrated plastic clip 46 of the appropriate size, or
with a trailing edge comb and mask made of plastic. The masked
portion 44 may instead be covered with a maskant 47 applied to the
surface of the article, such as a lacquer, tape, or wax. The
masking of the masked portion 44 defines the masked portion 44 and
an unmasked portion 48.
A layer of a noble metal, which may be platinum or other noble
metal such as palladium or rhodium, is deposited, numeral 56, onto
the airfoil 22, which thereby serves as a substrate 70. FIG. 4
illustrates the resulting structure, after the mask has been
removed. The platinum layer 72 overlies the unmasked portion 48.
There is no platinum layer overlying the masked portion 44. The
deposition is preferably accomplished by placing a
platinum-containing solution into a deposition tank and depositing
platinum from the solution onto the substrate 70, which is the
airfoil 22. In the deposition, the platinum layer 72 is deposited
onto the unmasked portion 48 of the substrate 70, but not only the
masked portion 44. An operable platinum-containing aqueous solution
is Pt(NH.sub.3).sub.4 HPO.sub.4 having a concentration of about
4-20 grams per liter of platinum, and the voltage/current source is
operated at about 1/2-10 amperes per square foot of facing article
surface. The platinum layer 72, which is preferably from about 1 to
about 6 micrometers thick and most preferably about 5 micrometers
thick, is deposited in 1-4 hours at a temperature of
190-200.degree. F.
After the platinum layer 72 is deposited, the mask is removed,
numeral 58. Where the mask is a separate article such as the
plastic clip 46, it is simply lifted away. Where the mask is an
overlay maskant 47 such as a lacquer, tape, or wax, it is stripped
away mechanically, chemically with a solvent, or physically by
melting.
An aluminum-containing layer 74 is deposited, numeral 60, overlying
both the (previously) masked portion 44 and the unmasked portion
48. The aluminum layer 38 is deposited by any operable approach,
with vapor deposition preferred. In that approach, a hydrogen
halide gas, such as hydrogen chloride or hydrogen fluoride, is
contacted with aluminum metal or an aluminum alloy to form the
corresponding aluminum halide gas. Other elements may be doped into
the aluminum layer from a corresponding gas, if desired. The
aluminum halide gas contacts the airfoil 22, depositing the
aluminum thereon. The deposition occurs at elevated temperature
such as from about 1825.degree. F. to about 2050.degree. F. during
a 4 to 20 hour cycle. The aluminum layer 74 is preferably from
about 12 to about 125 micrometers thick. The deposition technique
allows alloying elements to be co-deposited into the aluminum layer
74 if desired, from the halide gas. In this process, the aluminum
layer 38 is also deposited on the convex side 32. Such deposition
of aluminum on the convex side 32 is not harmful, and in fact forms
a beneficial diffusion aluminide layer on the convex side 32 which
resists oxidation in this less demanding region of the article.
Aluminum is inexpensive.
Because the deposition of aluminum is performed at elevated
temperature, the deposited aluminum atoms interdiffuse with the
platinum layer 34 (or interdiffused platinum/substrate region) and
the material of the substrate 70, numeral 62, forming a diffusion
zone 76. A diffusion subzone 78 formed from interdiffusion of the
platinum layer 72, the aluminum layer 74, and the substrate 70
contains platinum, aluminum, and elements found in the substrate,
primarily nickel because nickel is the primary component of the
substrate. A diffusion subzone 80 formed from interdiffusion of the
aluminum layer 74 and the substrate 70 contains aluminum and
elements found in the substrate, primarily nickel because nickel is
the primary component of the substrate. A significant amount of
interdiffusion of the layers is achieved during the aluminum
deposition step 60. Additional interdiffusion may be accomplished
if desired by maintaining the structure at elevated temperature
after the flow of halide gas is discontinued.
If further protection is required because the airfoil is to be used
at very high temperatures, a ceramic layer 82 may be deposited,
numeral 64, overlying the aluminum-containing layer 74. The ceramic
layer 40 is typically applied only over the concave side 30 and
thence over the interdiffused platinum-aluminide coating, but it
could be applied over the convex side 32 if desired. The ceramic
layer 82 may be applied by any operable technique, with electron
beam physical vapor deposition (EB-PVD) being preferred for the
preferred yttria-stabilized zirconia coating. The EB-PVD processing
may be preceded and/or followed by high-temperature processes that
may affect the distribution of elements in the bond coat. The
EB-PVD process itself is typically conducted at elevated
temperatures.
The final protected article has a platinum aluminide coating
covering the unmasked area 48, which in the preferred case is most
of the concave side 30 of the airfoil 22. A (non-platinum)
aluminide, often termed a nickel aluminide, covers the masked area
48, which in the preferred case is one of the portions 40 or 42
adjacent to the trailing edge 36 of the concave side 30.
Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications and enhancements may be made without departing from
the spirit and scope of the invention. Accordingly, the invention
is not to be limited except as by the appended claims.
* * * * *